Please keep in mind that the following arguments are my personal view from my experience in 7DD and everyone can agree or disagree with it.
All 3 heroes are really good tanks (probably amonst top 5), don’t get me wrong.
Kunchen: Yes, he’s beefy. But he’s just too slow to be a top war tank imo. He gives you too much time to work the board and is rather easy to beat, even with a purple team.
BK: Definitely a very good tank and even beefier stats than Kunchen. Bring a dispeller and his special skill (aka his Taunt) is gone. Yes, you might get one attack with 1 damage but that’s about it. Also, blue heroes are probably the strongest heroes in E&P.
Ursena: Very difficult tank indeed. Lots of people struggle against her. Her stats are the least tanky of all 4 heroes tho. She wrecks you on a bad board for sure, that’s what high damage dealing tanks do (so does GM). However, if you get a decent board and she manages to fire once it’s not over (yet). Have a heal ready (ideally a dispeller as well) and you can heal her damage and remove her reflect. Ofc this presumes you get the tiles to charge your healer and dispeller shortly after she fired. Clearing tiles into her is not a good idea. Hence, you should clear tiles into the flanks/corners and charge your heroes that way, if possible. The strategy is to avoid/ignore her as much as possible and don’t send tiles into her. If you bring non-yellows you don’t even have to worry about her reflect.
Guin: Tanky stats. Less tanky than Kunchen and BK, but more tanky than Ursena. The strategy against her is the exact opposite to Ursena. Throw bad tiles into her until you clear the board and it turns to your favor. Yes, she dies with 5 purple tiles. So does Ursena die with 5 yellow tiles tho.
The one thing Guin is superior tho imo is that her special is inevitable (except you bring Seshat or managed to fire Poseidon or Red Hood before she fires). Ursena’s damage can be healed and her reflect can be dispelled or ignored. However, Guin’s mana reduction cannot be dispelled or healed or anything alike - it just hits you.
Mana is a big success factor of each fight. Yes, you can win just by tile damage. But let’s be honest, if you have a board that lets you win just by tile damage the defending heroes don’t matter. You win against anyone if your board is that good. I’ve seen it many times where your heroes are ready to fire and Guin goes off right before that. From there on you’re always one step behind and play catch up if you don’t have a good follow up match to re-charge your heroes before she goes off again.
Yes, I feel comfortable fighting Guin (more than fighting Ursena) because I know how to handle her. Nevertheless, that doesn’t change how punishing and dangerous she is in wars, especially when you have to fight her with off-color teams.